Many components used in optical and semiconductor devices must be very smooth, since surface roughness acts to both scatter light and enhance absorption. For many multilevel integrated circuits, the planarization of dielectric layers, which acts to isolate devices as well as form the base layer for subsequent stacks of devices, is an important technology. Particularly in the case of the epitaxial growth of electronic materials, high quality substrates for many uses are expected to be atomically smooth.
The development of robust planarization techniques for III-nitride materials such as GaN will undoubtedly play a crucial role in the continued advancement of nitride-based semiconductor devices. With the inevitable emergence of commercially available GaN wafers, manufactures of such have an unquestionable need to develop methods to planarize them prior to sale. Such substrates would bypass problems such as cracking and high dislocation densities stemming from the epitaxial growth of nitrides on non-native substrates.
The growth of thick, atomically smooth GaN films is a significant challenge when employing high growth rates (on the order of hundreds of microns per hour) obtained with techniques such as halide vapor phase epitaxy (HVPE). The ability to planarize rough films would allow their use in the epitaxial growth of devices and thus increase the yield of the HVPE process. Substrate manufacturers lose material due to rough areas resulting from process non-uniformities. Appropriate planarization methods would act to recover such lost area.
The traditional method of planarizing semiconductor materials is lapping followed by chemo-mechanical polishing (CMP). Lapping is a coarse planarization technique utilizing successively smaller sizes of pad-embedded grits of very hard materials such as diamond, cubic boron nitride, or alumina to achieve bulk removal and flattening of a material through grinding action. Lapped surfaces tend to have micron-scale roughness. Chemo-mechanical polishing is a planarization technique that simultaneously uses both chemical and mechanical processes to achieve a finer degree of planarization. This is accomplished by a far gentler grinding of the semiconductor with nanoscale grits embedded in an etching solution. CMP techniques exist for materials such as silicon, germanium, arsenide and phosphide semiconductors.
III-nitrides materials, however, do not lend themselves to wet etching at room temperature due to their extreme hardness and resistance to chemical attack. The Vicker hardness of GaN is 12 GPa, which is close to that of sapphire, which has a hardness of 20 GPa. This places GaN at level 9 on the Moh's hardness scale. Efforts to polish GaN with harder materials such as diamond typically leave the surface full of undesirable scratches. Furthermore, since GaN is highly anisotropic, every crystal face presents its own challenges. Weyher, et al. (Journal of Crystal Growth, 182:17 (1997)) report the polishing of freestanding GaN platelets using KOH solutions (1:2-1:20) and a soft polishing pad with an applied pressure of 2-4 kg/cm2. However, that work is applicable only to polishing of the nitrogen polarity of GaN, which is by far the easiest orientation to etch. Tavernier et al. (Electrochemical and Solid State Letters, 5:G61 (2002)) concluded that polishing action was not achieved on Ga-polar material.
GaN ablation experiments, utilizing a pulsed KrF excimer laser under appropriate conditions, have also been reported as a method to decrease surface roughness. While laser etching of GaN is feasible, the process of globally planarizing large areas of very rough material by this method would be cumbersome and time consuming, owing to the need for precise control of mechanical stages, optical alignment, and feedback systems to allow for adaptation of the local pulse dosage to changes in the surface height.
Etch back methods have been used during the processing of multilevel integrated circuits to planarize interlayer dielectrics. However, such methods have not been applied to III-nitride materials due to the difficulties in etching these materials as described above.
Thus, there remains a need for a robust, generally applicable planarization technique for GaN and other III-nitrides.